X-ray computed tomography apparatus

ABSTRACT

According to one embodiment, an X-ray computed tomography apparatus comprises an X-ray generating unit, an X-ray detecting unit, a rotating mechanism configured to rotate the X-ray generating unit and the X-ray detecting unit, a reconstruction processing unit configured to reconstruct first images respectively corresponding to volumes, a shift detecting unit configured to detect shifts of an object image due to warp of the top within a reconstruction coordinate system of the reconstruction processing unit, which shifts respectively correspond to the first images, and a control unit configured to move an origin of the reconstruction coordinate system for each of the volumes based on each of the detected shifts and control the reconstruction processing unit to reconstruct second images, which shifts respectively correspond to the volumes from the output from the X-ray detecting unit on the reconstruction coordinate system whose origin has been moved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2009-113571, filed May 8, 2009; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to an X-ray computedtomography apparatus.

BACKGROUND

Many of the beds provided for X-ray computed tomography apparatuses usea structure configured to cantilever a top on which an object is placed.This structure allows a reduction in installation space and hassuperiority in enlarging a work area for an operator. However, the topis inevitably subjected to so-called “warp”, that is, deforms downwarddue to its own weight and the weight of an object.

Conventional methods have used physical and mechanical measures forsuppressing the warp of the top, e.g., using a mechanism for supportingthe top and improving the rigidity of the top itself. These measures canreduce the warp of the top due to the gravity of the earth but cannotavoid the warp because of the structure configured to cantilever thetop.

Owing to this problem, when volume scanning is repeated while the scanposition is changed along, for example, the rotation axis of the X-raytube (Z-axis; approximate to the body axis of an object), an overallimage obtained by synthesizing scanned images along the Z-axis hasapparent differences in level at the volume scan switching positions. Asa consequence, the overall image becomes discontinuous.

In order to solve this problem, a method of measuring the relationshipbetween the position of a top and a warp amount in advance and shiftingan image position in accordance with the measurement is often used.These solutions involve a problem of requiring preliminary scanning. Themethod of solving the problem by shifting images involves a problem ofincapability of substantially solving the problem of differences inlevel (discontinuity) because the warp amount of the top is smaller thanthe pixel pitch or is not an integer multiple of the pixel pitch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the arrangement of an X-ray computed tomographyapparatus according to this embodiment;

FIG. 2 is a perspective view of a gantry and bed in FIG. 1;

FIG. 3 is a view showing scan regions (three-dimensional regions)preliminarily scanned for top position correction processing in thisembodiment;

FIG. 4 is a flowchart showing a procedure for top position correctionprocessing according to this embodiment;

FIG. 5 is a view showing an x-direction profile associated with theintegral values obtained by integrating the CT values of a top region inthe Y direction in top position detection processing in step S12 in FIG.4;

FIG. 6 is a view showing the top position shift amount of a volume (n+1)relative to a volume (n) in shift amount measurement processing in stepS13 in FIG. 4;

FIG. 7 is a supplementary view of FIG. 6;

FIG. 8 is a supplementary view for a specific description of acalculation method in the shift amount measurement processing in stepS13 in FIG. 4;

FIG. 9 is a view comparatively showing the continuity of a tomogram inthe Z direction after top position correction in this embodiment and thediscontinuity of a tomogram in the Z direction before top positioncorrection; and

FIG. 10 is a view showing the shift amount of the top position on aslice basis in the shift amount measurement processing in step S13 inFIG. 4.

DETAILED DESCRIPTION

In general, according to one embodiment, an X-ray computed tomographyapparatus comprises:

a bed including a cantilevered top;

an X-ray generating unit configured to generate X-rays;

an X-ray detecting unit configured to detect X-rays transmitted throughan object placed on the top;

a rotating mechanism configured to rotate the X-ray generating unit andthe X-ray detecting unit about a rotation axis;

a reconstruction processing unit configured to reconstruct a pluralityof first images respectively corresponding to a plurality of volumes ora plurality of slices arrayed along the rotation axis from an outputfrom the X-ray detecting unit;

a shift detecting unit configured to detect shifts of an object imagedue to warp of the top within a reconstruction coordinate system of thereconstruction processing unit, which shifts respectively correspond tothe plurality of first images, based on the plurality of first images;and

a control unit configured to move an origin of the reconstructioncoordinate system for each of the volumes or each of the slices based oneach of the detected shifts and control the reconstruction processingunit to reconstruct a plurality of second images, upon correction of theshifts of the object image, which shifts respectively correspond to theplurality of volumes or the plurality of slices from the output from theX-ray detecting unit on the reconstruction coordinate system whoseorigin has been moved.

An X-ray computed tomography apparatus according to a preferredembodiment of the present invention will be described in detail below.Note that X-ray computed tomography apparatuses include various types ofapparatuses, e.g., a rotate/rotate-type apparatus in which an X-ray tubeand an X-ray detector rotate together around an object, and astationary/rotate-type apparatus in which many detection elements arearrayed in the form of a ring, and only an X-ray tube rotates around anobject. The present invention can be applied to either type. In thiscase, the rotate/rotate type will be exemplified. In order toreconstruct image data, projection data corresponding to one rotationaround an object, i.e., 360° , is required, or (180° +fan angle)projection data is required in the half scan method. The presentinvention can be applied to either of these reconstruction schemes. The360° method will be exemplified. As mechanisms of converting incidentX-rays into electric charges, the following techniques are themainstream: an indirect conversion type that converts X-rays into lightthrough a phosphor such as a scintillator and converts the light intoelectric charges through photoelectric conversion elements such asphotodiodes, and a direct conversion type that uses generation ofelectron-hole pairs in a semiconductor such as selenium by X-rays andmigration of the electron-hole pairs to an electrode, i.e., aphotoconductive phenomenon. As an X-ray detection element, either ofthese schemes can be used. Recently, with advances toward thecommercialization of a so-called multi-tube type X-ray CT apparatushaving a plurality of pairs of X-ray tubes and X-ray detectors mountedon a rotating ring, related techniques have been developed. The presentinvention can be applied to both a conventional single-tube type X-rayCT apparatus and a multi-tube type X-ray CT apparatus. The single-tubetype X-ray CT apparatus will be exemplified here.

FIG. 1 is a view showing the arrangement of the X-ray computedtomography apparatus according to this embodiment. FIG. 2 is aperspective view of the gantry and bed in FIG. 1. A bed 121 includes atop 120 on which an object is placed. A moving mechanism 122 supportsthe top 120 so as to make it movable along the Z-axis. The movingmechanism 122 cantilevers the top 120. That is, the top 120 is supportedat one end, and the other end of the top 120 is not supported. The otherend of the top 120 is free.

A gantry 100 accommodates a rotating support mechanism. The rotatingsupport mechanism includes a rotating ring 102 and a ring supportmechanism which supports the rotating ring 102 so as to make itrotatable about the Z-axis. The rotating ring 102 is equipped with anX-ray tube 101. The X-ray tube 101 receives a tube voltage and a tubecurrent from a high voltage generator 109 via a slip ring 108 andgenerates X-rays from the focal point. A collimator unit 118 is attachedto the X-ray irradiation window of the X-ray tube 101. The collimatorunit 118 limits X-rays from the X-ray tube 101 to, for example, arectangular shape. A collimator unit 111 forms X-rays into a cone beamshape (pyramidal shape).

The rotating ring 102 is equipped with an X-ray detector 103. The X-raydetector 103 faces the X-ray tube 101 through the Z-axis. The X-raydetector 103 includes a plurality of X-ray detection elements.Typically, a single X-ray detection element forms a single channel. Aplurality of X-ray detection elements are arrayed in a two-dimensionalpattern. In addition, a plurality of X-ray detection elements arearranged in a plurality of X-ray detection element arrays each having aplurality of X-ray detection elements arrayed in a line. The X-raydetector 103 may comprise a single X-ray detection element array. Inimaging or scanning operation, an object is inserted into a cylindricalimaging area between the X-ray tube 101 and the X-ray detector 103.

A data acquisition circuit 104, which is generally called a DAS (DataAcquisition System), is connected to the output of the X-ray detector103. The data acquisition circuit 104 is provided with, for eachchannel, an I-V converter for converting the current signal obtained viaeach channel of the X-ray detector 103 into a voltage, an integrator forperiodically integrating these voltage signals in synchronism with anX-ray irradiation period, an amplifier for amplifying an output signalfrom the integrator, and an analog/digital converter for converting anoutput signal from the amplifier into a digital signal.

The data (pure raw data) output from the data acquisition circuit 104 istransmitted to a preprocessor 106 via a noncontact data transmissionunit 105 using magnetic transmission/reception or opticaltransmission/reception. The preprocessor 106 preprocesses this pure rawdata. The preprocessing includes, for example, sensitivity nonuniformitycorrection processing between channels and the processing of correctingan extreme decrease in signal intensity or signal omission due to anX-ray absorber, mainly a metal portion. The data (called raw data orprojection data; projection data in this case) output from thepreprocessor 106 immediately before reconstruction processing is storedin a projection data storage unit 112 including a magnetic disk,magneto-optical disk, or semiconductor memory in association with datarepresenting view angles at the time of data acquisition.

Note that projection data reflects the intensity of X-rays attenuated byan object. Projection data are repeatedly acquired during one rotationof the X-ray tube 101. A position at which projection data are acquiredis called a view. A set of projection data throughout all channelscorresponding to the respective views is called a projection data set.The respective view angles are represented by angles in the range of 0°to 360° which represent the respective positions on a circular orbitcentered on a rotation central axis Z along which the X-ray tube 101revolves, with the position of the uppermost position on the orbit being0°. Each channel data of a projection data set is identified by a viewangle, a cone angle, and a channel number.

A reconstruction processing unit 114 reconstructs a plurality of images(three-dimensional images) by the Feldkamp method or the cone beamreconstruction method based on a plurality of projection data setsacquired in the range of 360° or 180° +fan angle. The plurality ofthree-dimensional images respectively correspond to a plurality ofvolumes in an almost cylindrical shape which are arrayed along therotation axis. The reconstruction processing unit 114 also reconstructsa plurality of images (two-dimensional images) associated with aplurality of almost circular slices arrayed along the rotation axis by,for example, the fan beam reconstruction method (also called the beamconvolution back projection method). The Feldkamp method is areconstruction method to be used when projection rays intersect areconstruction plane like a cone beam. In the Feldkamp method,convolution processing is performed by regarding a projection beam as afan projection beam on the premise that the cone angle is small, whereasback projection processing is performed along an actual ray in scanningoperation. The cone beam reconstruction method is a reconstructionmethod which corrects projection data in accordance with the angle of aray relative to a reconstruction plane as a method which suppresses coneangle errors more than the Feldkamp method. For reconstructionprocessing, a reconstruction coordinate system expressed by threeorthogonal axes (x, y, z) corresponding to a real space coordinatesystem (X, Y, Z) is used.

In practice, projection data corresponding to the respective points on areconstruction coordinate system are specified by view angles, coneangles, and channel numbers, and the correspondence relationship betweenthe respective points on the reconstruction coordinate system and viewangles, cone angles, and channel numbers of the corresponding projectiondata is determined in advance. The resultant data are stored in a ROM.Changing read control on this ROM will arbitrarily change thecorrespondence relationship between the respective points on thereconstruction coordinate system and the view angles, cone angles, andchannel numbers of the corresponding projection data, therebysubstantially implementing the movement of the origin of thereconstruction coordinate system.

This embodiment measures the shift amounts of an object image from areference position due to the warp of the top 120 within thereconstruction coordinate system (image) of the reconstructionprocessing unit 114, which shift amounts respectively correspond to aplurality of images, based on a plurality of images respectivelycorrespond to a plurality of volumes or slices. These shift amountsincrease depending on the distance from the top supporting position. Ashift amount is typically measured based on the top position identifiedby the top region extracted from each image (first image). However, ashift amount may be measured based on the boundary identified betweenthe object region extracted from an image and the top region.Alternatively, a shift amount may be measured based on, typically, thespinal cord region of an object which is extracted from an image. Thefollowing description is based on the assumption that a shift amount ismeasured based on a top region.

A top position detecting unit 113 extracts a local region having apredetermined size which includes the short-axis central point of thetop from a two-dimensional image (tomogram) associated with an x-y planecrossing the top which is generated from a three-dimensional image. TheY-axis of a two-dimensional image corresponds to the warping directionof the top. The top position detecting unit 113 integrates CT values inthe X-axis direction corresponding to the short-axis direction of thetop for each of all the Y-coordinate points in the local region. Changesin integral value in the Y-axis direction will be referred to as anintegral value profile. The top position detecting unit 113 specifiesthe maximum value of the integral value profile, and detects thecorresponding Y-coordinate as a top position on the corresponding slicein the warping direction of the top. The top position may be specifiedfor the position in which the median ((max-min)/2) is shown on theintegral value profile or the position in which a predetedminedthreshold value is shown on the integral value profileis.

A shift amount measuring unit 117 measures the distance between the topposition detected by the top position detecting unit 113 and thereference position as a shift amount. Typically, when a plurality ofthree-dimensional regions discretely or continuously ranging along therotation axis almost parallel to the body axis of the object arescanned, the shift amount measuring unit 117 sets a reference positionat the top position detected from a two-dimensional image in a specificone of the plurality of three-dimensional regions which is nearest tothe top supporting position.

A shift amount correction unit 119 substantially corrects the originposition on the reconstruction coordinate system based on the shiftamount measured by the shift amount measuring unit 117 under the controlof a host computer 110. The reconstruction processing unit 114 thenreconstructs a three-dimensional image. The shift amount correction unit119 moves the origin of the reconstruction coordinate system for eachvolume or slice based on each shift amount of the top which is measuredfor each volume or slice. The shift amount correction unit 119 mayperform processing almost equivalent to the above processing as follows.The shift amount correction unit 119 corrects the shift of the topposition by changing the read address of projection data to be read outfrom the projection data storage unit 112 to the reconstructionprocessing unit 114 based on the shift amount measured for each volumeor slice under the control of the host computer 110.

FIG. 3 shows a plurality of volume regions (a plurality ofthree-dimensional regions) VS obtained by volume scanning in thisembodiment. FIG. 4 shows a procedure for top position shift correctionprocessing in the embodiment. As shown in FIG. 3, the X-ray tube 101 andthe X-ray detector 103 continuously rotate around an object under thecontrol of the host computer 110. During this continuous rotation,conventional scanning is repeated a plurality of number of times whilethe scan position is discretely displaced. Conventional scanning isperformed such that the top is repeatedly moved and stopped, andacquisition of at least a 360° projection data set at each stop positionis executed at least once, unlike helical scanning which is performedsuch that a projection data set is repeatedly acquired while the topcontinuously moves concurrently with the continuous rotation of theX-ray tube 101 and X-ray detector 103. A projection data set isintermittently repeated while the top is moved by a predetermineddistance and stopped. The moving distance of the top per scan istypically matched with the width of the three-dimensional region VS,which can be reconstructed by one scan, in the Z-axis direction. In thiscase, a plurality of three-dimensional regions VS(1) to VS(4) arecontinuous. Obviously, the moving distance of the top per scan may beshorter than the width of the three-dimensional region VS in the Z-axisdirection. In this case, the plurality of three-dimensional regionsVS(1) to VS(4) partially overlap. In addition, the plurality ofthree-dimensional regions VS(1) to VS(4) have gaps.

In this case, the projection data sets acquired by repetitiveconventional scanning are shared by top position correction processingand reconstruction processing for a final diagnosis image, and there isno need to perform scanning for top position correction processing.

When all the projection data sets associated with all thethree-dimensional regions VS(1) to VS(4) are completely acquired byrepetitive conventional scanning, image reconstruction processing isperformed (S11). In the image reconstruction processing, for each of thethree-dimensional regions VS(1) to VS(4), the reconstruction processingunit 114 reconstructs three-dimensional image data (volume data)Volume(1) to Volume(4) in almost cylindrical reconstruction ranges(regions equivalent to VS1 to VS4) typically by a cone beamreconstruction method based on projection data sets in the view anglerange of 360° or 180° +fan angle, as shown in FIG. 8. Thisreconstruction processing is performed on the reconstruction coordinatesystem expressed by the reconstruction processing unit 114 usingexisting three orthogonal axes (x, y, z).

The top position detecting unit 113 generates two-dimensional images(tomograms) 2Ds(1) to 2Ds(4) associated with slice positions in thethree-dimensional regions VS(1) to VS(4) which are nearest to thecantilevering position of the top and two-dimensional images 2De(1) to2De(4) associated with slice positions in the three-dimensional regionsVS(1) to VS(4) which are farthest from the cantilevering position of thetop from the volume data Volume(1) to Volume(4) respectivelycorresponding to the three-dimensional regions VS(1) to VS(4).

According to the above description, in step S11, the reconstructionprocessing unit 114 reconstructs the volume data Volume(1) to Volume(4)and generates the two-dimensional images 2Ds(1) to 2Ds(4) and 2De(1) to2De(4) by slice conversion processing for the respective volume datafrom Volume(1) to Volume(4). However, the reconstruction processing unit114 may directly reconstruct the two-dimensional images 2Ds(1) to 2Ds(4)and 2De(1) to 2De(4) by the fan beam reconstruction method or thefiltered back projection method for slices at predetermined positions inthe three-dimensional regions VS(1) to VS(4) without reconstructing thevolume data Volume(1) to Volume(4).

The top position detecting unit 113 extracts a top region from each ofthe two-dimensional images 2Ds(1) to 2Ds(4) and 2De(1) to 2De(4) bythreshold processing. As shown in FIG. 5, the top position detectingunit 113 then extracts a local region having a predetermined size whichincludes the short-axis central point of the extracted top region. Thetop position detecting unit 113 integrates CT values associated with theX-axis direction corresponding to the short-axis direction of the topfor each of all the Y-coordinate points in the local region. FIG. 5exemplifies changes in integral value in the Y-axis direction as anintegral value profile. The top position detecting unit 113 specifiesthe maximum value of each integral value profile, and detects theY-coordinates as top positions Ys1 to Ys4 and Ye1 to Ye4 on thecorresponding slices in the top warping direction. The top position maybe specified for the position in which the median ((max-min)/2) is shownon the integral value profile or the position in which a predetedminedthreshold value is shown on the integral value profileis.

The shift amount measuring unit 117 measures the distance between thetop position detected by the top position detecting unit 113 and thereference position as a shift amount (S13). Typically, as shown in FIG.7, a position to be set as a reference position is the top position Ye1detected from the two-dimensional image 2De(1) in the specificthree-dimensional region VS(1), of the plurality of three-dimensionalregions, which is nearest to the top supporting position. As shown inFIGS. 6 and 8, the shift amount measuring unit 117 measures the shiftamounts of the top positions in the three-dimensional regions VS(2) toVS(4) with reference to the top position in the three-dimensional regionVS(1). The shift amount measuring unit 117 calculates shift amountsCor(2) to Cor(4) of the top positions in the three-dimensional regionsVS(2) to VS(4) as follows:

Cor(2)=Ye0−Ys1

Cor(3)=(Ye1−Ys2)+Cor(2)

Cor(4)=(Ye2−Ys3)+Cor(3)

That is, with regard to the adjacent first and second three-dimensionalregions, it is possible to eliminate or reduce differences in level atthe top positions on a synthesized image obtained by synthesizinglong-axis images of X-Z planes in the respective three-dimensionalregions by cumulatively adding the shift amount between the top positionYe at the trailing end of the first three-dimensional region and the topposition Ys at the leading end of the second three-dimensional region tothe initial shift amount Cor(2).

The shift amount correction unit 119 substantially corrects the originposition on the reconstruction coordinate system of the reconstructionprocessing unit 114 based on the shift amounts Cor(2) to Cor(4) measuredby the shift amount measuring unit 117. The reconstruction processingunit 114 then reconstructs a three-dimensional image (S14). Morespecifically, this operation is equivalent to changing the read addressof projection data to be read out from the projection data storage unit112 to the reconstruction processing unit 114, based on the shiftamounts measured by the shift amount measuring unit 117, so as tocorrect the top position shifts.

Measuring and correcting top position shifts before and after therespective three-dimensional regions in this manner will generatelong-axis images of X-Z planes in the respective three-dimensionalregions from four three-dimensional images having undergone top positioncorrection. On the synthesized image obtained by synthesizing theseimages, it is possible to eliminate or reduce differences in level atthe top positions, as shown in FIG. 9, while allowing the warp of thetop in each three-dimensional region.

This embodiment is configured to re-execute reconstruction processing bysubstantially shifting the origin in reconstruction processing insteadof an attempt to eliminate shifts by simply shifting initiallyreconstructed three-dimensional images. Assume that initiallyreconstructed three-dimensional images are simply shifted. In this case,when a shift amount is smaller than the pixel pitch or does not match aninteger multiple of the pixel pitch, the accuracy of positional shiftcorrection deteriorates. This may substantially increase positionalshifts. In contrast, this embodiment can avoid such deterioration inaccuracy by re-executing reconstruction processing by substantiallyshifting the origin in reconstruction processing.

In the prior art, there is a limit to the suppression of physical warp,and, for example, an increase in the strength of the top may cause othertroubles (artifacts and noise). In addition, since a warp amount isexpected to change due to various factors such as a scan range, anobject position, and an object weight, it is impossible to prepare acorrection table. In addition, these conventional methods require muchefforts and time, e.g., scanning twice. In contrast, this embodiment cansolve the above problems, eliminate patient position shifts in therespective CT images, and save the user a lot of labor.

According to the above description, the positional shifts betweenvolumes are measured and corrected on a volume basis. However, thepositional shifts between central slices of volumes may be measured andcorrected on a volume basis.

According to the above description, the positional shifts betweenvolumes are measured and corrected on a volume basis. As shown in FIG.10, however, the positional shifts between a plurality of slicesconstituting each volume may be measured and corrected on a slice basis.Alternatively, it is possible to correct positional shifts on apredetermined number of slices basis instead of correcting positionalshifts on a slice basis.

In addition, according to the above description, volume scanning hasbeen exemplified as a scanning method. However, shift correctionaccording to this embodiment can also be applied to helical scanning. Inthis case, positional shifts are corrected every slice or every apredetermined number of slices.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

1. An X-ray computed tomography apparatus comprising: a bed including acantilevered top; an X-ray generating unit configured to generateX-rays; an X-ray detecting unit configured to detect X-rays transmittedthrough an object placed on the top; a rotating mechanism configured torotate the X-ray generating unit and the X-ray detecting unit about arotation axis; a reconstruction processing unit configured toreconstruct a plurality of first images respectively corresponding to aplurality of volumes or a plurality of slices arrayed along the rotationaxis from an output from the X-ray detecting unit; a shift detectingunit configured to detect shifts of an object image due to warp of thetop within a reconstruction coordinate system of the reconstructionprocessing unit, which shifts respectively correspond to the pluralityof first images, based on the plurality of first images; and a controlunit configured to move an origin of the reconstruction coordinatesystem for each of the volumes or each of the slices based on each ofthe detected shifts and control the reconstruction processing unit toreconstruct a plurality of second images, upon correction of the shiftsof the object image, which shifts respectively correspond to theplurality of volumes or the plurality of slices from the output from theX-ray detecting unit on the reconstruction coordinate system whoseorigin has been moved.
 2. The apparatus according to claim 1, whereinthe shift detecting unit detects a shift of the object image based on atop region extracted from each of the first images.
 3. The apparatusaccording to claim 2, wherein a position at which an integral valueobtained by integrating pixel values within the extracted top region ina short-axis direction of the top exhibits a maximum value is detectedas a position of the object image.
 4. The apparatus according to claim1, wherein the shift detecting unit detects a shift of the object imagebased on a boundary between an object region and a top region which areextracted from each of the first images.
 5. The apparatus according toclaim 1, wherein the shift detecting unit detects a shift of the objectimage based on a spinal cord region of the object extracted from each ofthe first images.
 6. The apparatus according to claim 1, wherein thex-ray detecting unit comprises a plurality of X-ray detection elementsarrayed two-dimensionally.
 7. The apparatus according to claim 6,wherein the shift detecting unit detects a shift of the object image byusing a plurality of two-dimensional images respectively correspondingto two end portions of each of the volumes.
 8. The apparatus accordingto claim 6, wherein the shift detecting unit detects a shift of theobject image by using at least one two-dimensional image correspondingto a central portion of the volume.
 9. The apparatus according to claim1, wherein the shift detecting unit detects the shift by setting, as areference position, the object image of one of the plurality of firstimages which is nearest to a supporting position of the top.
 10. Theapparatus according to claim 1, wherein the shift detecting unit detectsthe shift by setting, as a reference position, the object image of oneof the plurality of first images which is located at a middle thereof.11. An image reconstruction processing apparatus comprising: a storageunit configured to store projection data respectively corresponding to aplurality of arrayed volumes or a plurality of arrayed slices associatedwith an object; a reconstruction processing unit configured toreconstruct a plurality of first images respectively corresponding tothe plurality of volumes or the plurality of slices based on the storedprojection data; a shift detecting unit configured to detect shifts ofan object image due to warp of the top within a reconstructioncoordinate system of the reconstruction processing unit, which shiftsrespectively correspond to the plurality of first images, based on theplurality of first images; and a control unit configured to move anorigin of the reconstruction coordinate system for each of the volumesor each of the slices based on each of the detected shifts and controlthe reconstruction processing unit to reconstruct a plurality of secondimages, upon correction of the shifts of the object image, which shiftsrespectively correspond to the plurality of volumes or the plurality ofslices from the output from the X-ray detecting unit on thereconstruction coordinate system whose origin has been moved.
 12. Animage reconstruction processing method comprising: storing projectiondata respectively corresponding to a plurality of arrayed volumes or aplurality of arrayed slices associated with an object; reconstructing aplurality of first images respectively corresponding to the plurality ofvolumes or the plurality of slices based on the stored projection data;detecting shifts of an object image due to warp of the top within areconstruction coordinate system, which shifts respectively correspondto the plurality of first images, based on the plurality of firstimages; moving an origin of the reconstruction coordinate system foreach of the volumes or each of the slices based on each of the detectedshifts; and reconstructing a plurality of second images, upon correctionof the shifts of the object image, which shifts respectively correspondto the plurality of volumes or the plurality of slices based on thestored projection data on the reconstruction coordinate system whoseorigin has been moved.